The head of the agency responsible for airline security in the US has resigned after warnings that aircraft are still at risk from terrorist attack.
Stone was only in the job for eight months
Transportation Security Administration (TSA) chief David Stone quit after just eight months in the job.
He is the third TSA boss to resign since the 11 September 2001 attacks.
Last month, an official US report said airliners were still vulnerable to terrorism, despite $12bn spent by the TSA on improving security.
The TSA gave no reason for Mr Stone's departure.
'Holes and weaknesses'
In a brief statement, TSA spokesman Mark Hatfield said: "Admiral Stone has informed [Homeland Security] Secretary [Michael] Chertoff of his intention to step down from TSA and has agreed to the department's request to remain until June to assist with the transition of a successor."
It follows the revelation in March that a confidential report compiled by the Department of Homeland Security and the Federal Bureau of Investigation (FBI) concluded airlines remained at risk to terrorist attacks, despite a raft of security improvements.
The report said commercial airliners were ''likely to remain a target and a platform for terrorists,'' and that the al-Qaeda terror network appeared determined to study and test new American security measures to ''uncover weaknesses".'
It also warned that terrorists could target helicopters and corporate jets or private planes parked at small airports where security is relatively weak.
Another report, by the Coalition of Airline Pilots (CAP), warned of "gaping holes" in aviation security.
The CAP report said screening of airport staff and cargo had not improved in the wake of 11 September 2001.
"The technology exists, or could be updated, to address many of these security problems, but neither the airlines, the airports nor government officials have given these issues the priority they deserve," the report warned.
The TSA has spent more than $12bn since the 2001 attacks on improving security, including new baggage screeners, more air marshals and making cockpit doors bullet-proof.